In short. A generator data sheet is a promise in numbers. The headline kVA tells you very little on its own; the value is in the fields underneath — how the engine is rated, how the alternator behaves when a load is switched on, how much fuel it really uses at part load, and under what conditions every figure was measured. Two sets with the same kVA can differ widely in engine size, voltage dip, fuel use and life. This guide walks through each field in the order most Indian and international sheets present them, and shows what to check.
Who this page is for: For buyers, consultants, project engineers and purchase teams comparing generator offers: read every field of a data sheet with confidence, spot what is missing or weak, and compare two sheets fairly.
Key facts
- Every figure has a basis: rating class, power factor, reference conditions and tolerance. A number without its basis cannot be compared.
- The engine must be able to drive the alternator and its own cooling fan, with margin. Check engine kW against required kW.
- Transient performance — governing class, voltage dip, recovery time — decides whether motors start and UPS units stay happy.
- Fuel figures are test-bench values with a tolerance; part-load figures matter more than the full-load one for most sites.
- A family brochure is not a data sheet. Ask for the sheet that matches the exact model, alternator and options quoted.
Rating classes (standby, prime, continuous), site derating and the kW–kVA relationship are explained on our Datasheets & Brochures page. This page covers everything else on the sheet.
1. The genset block
| Field | What it tells you | What to check |
|---|---|---|
| Rated output (kVA / kWe) | Electrical output at the stated power factor, usually 0.8 lagging | Which rating class; kWe = kVA × 0.8; prime or standby must match how you will run it. |
| Rated current | Full-load current per phase | kVA × 1000 ÷ (1.732 × voltage). At 415 V this is about 1.39 A per kVA. A mismatch suggests a copy-paste error. |
| Voltage, frequency, phases | 415 V, 50 Hz, three-phase is standard in India | Correct for your site; voltage adjustment range. |
| Speed | Usually 1,500 rpm for 50 Hz four-pole sets | 3000 rpm sets are lighter and noisier; suited to small portable duty. |
| Governing class | Performance class for frequency and transient behaviour, G1 to G4 | G2 suits general loads; G3 is typically specified for telecom, data and UPS-heavy sites. “Mechanical governor” usually means a lower class. |
| Overload capability | For prime-rated sets, commonly 10 % overload for 1 hour in every 12 hours | Standby ratings normally have no overload allowance. |
| Noise level | dB(A) at a stated distance, usually 1 m | Distance and test conditions stated; compliant with the applicable noise limit for the enclosure. |
| Emissions compliance | The applicable Indian emission stage for new sets, with certificate | Certificate matches the engine and power rating quoted. |
2. The engine block
| Field | What it tells you | What to check |
|---|---|---|
| Engine gross / net power (kWm) | Mechanical power at the flywheel; net is after the cooling fan | Must exceed the power needed for the rated kWe — see the worked check below. |
| Cylinders, layout, displacement | Engine size; more displacement per kVA usually means lower stress | Compare litres per kVA between offers. |
| Bore × stroke, compression ratio | Engine geometry | Mainly for engineers; confirms the engine model. |
| Aspiration | Naturally aspirated, turbocharged, or turbocharged with aftercooling | Turbocharged engines give more power from less displacement but are more sensitive to light load and air-filter condition. |
| Fuel system | Mechanical pump or electronic (common rail) | Electronic engines need diagnostic tools and trained service. |
| Lube oil capacity, consumption, change interval | Service needs and engine health benchmark | Change interval stated in hours and months; oil consumption as a percentage of fuel. |
| Coolant capacity, radiator design ambient | Cooling margin | For Indian conditions, check the radiator is designed for high ambient — often 50 °C — with the enclosure fitted. |
| Combustion air, exhaust flow and temperature, allowable back-pressure | Data for the room and exhaust design | Needed for ventilation and exhaust pipe sizing. |
| Heat rejection | Heat to coolant and to the room | Needed for indoor installations. |
3. The alternator block
| Field | What it tells you | What to check |
|---|---|---|
| Frame / model and rating at the stated temperature rise | The alternator’s own capability | Alternator rating at least equal to the set rating — often larger where motor or UPS loads dominate. |
| Insulation class and temperature rise class | Insulation is often class H; the rise may be held to class F or H | An alternator running at class F rise with class H insulation has thermal margin and longer life. |
| Efficiency at 75 % and 100 % load | Losses | Used to check the engine power needed. |
| Voltage regulation | Steady-state voltage band, commonly about ±1 % with a good AVR | Basis: from no load to full load, at what power factor. |
| Excitation system | Self-excited (shunt), auxiliary winding, or permanent-magnet generator | PMG or auxiliary excitation sustains short-circuit current and handles non-linear loads better. |
| Short-circuit withstand | Commonly about 3 × rated current for 10 seconds with excitation support | Needed for downstream breakers to trip — without it the fault current can collapse. |
| Winding pitch | 2/3 pitch suppresses third-harmonic voltage | Machines run in parallel with linked neutrals should have the same pitch; mixing pitches circulates third-harmonic current in the neutral. |
| Waveform distortion, telephone harmonic factor | Quality of the voltage waveform | Typically under a few percent at no load. |
| Unbalanced load capability | How much phase unbalance it can carry continuously | Important for sites with large single-phase loads. |
| Reactances (X″d, X′d, Xd) and time constants | Fault current and voltage-dip behaviour | Needed for the fault-level study and protection settings. |
| Degree of protection | IP rating of the alternator, often IP23 | Higher where dust or moisture is severe. |
4. Transient performance — the fields that decide real-life behaviour
- Voltage dip on load acceptance — how far voltage falls when a block of load is switched on, and the block size used for the figure. A dip figure without the load step is meaningless.
- Frequency dip and recovery time — how far speed falls and how fast it recovers. These decide UPS and VFD behaviour.
- Maximum single load step — the largest block the set accepts in one step. Turbocharged engines often cannot take full load in a single step.
- Motor starting kVA — the starting kVA the alternator can supply at a stated voltage dip (for example at 15 % or 20 % dip).
- Time to rated voltage and frequency from start — matters for emergency loads.
Our Voltage Dip and Transient Response pages explain how to use these figures.
5. The fuel block
- Consumption at 50, 75 and 100 % load (sometimes 25 % too). For most standby and prime sites, the 50–75 % figures matter more than full load.
- Units and basis: litres per hour or grams per kWh; fuel density used; tolerance — often +5 %.
- Specific consumption check: litres per hour ÷ kW at that load. At full load, healthy modern diesel sets commonly fall in the region of about 0.24–0.30 L/kWh. A figure far below that deserves a question.
- Tank capacity and runtime: runtime at what load? Usable versus nominal volume?
6. Enclosure, controller and documents
- Enclosure: material and thickness, corrosion protection, acoustic rating, door locks, lifting and fork-lift points, base tank bunding.
- Dimensions and weight: both dry and wet (with fuel, oil and coolant) — the wet weight decides the foundation and the crane.
- Controller: AMF capability, measurements, protections, event log, communication port, remote monitoring.
- Breaker: type, rating, breaking capacity, trip unit.
- Standards stated: generating set performance standard, engine power standard, alternator standard, and the emission and noise certificates.
- Document identity: model, revision and date — and whether it matches the quotation.
Worked check: a 125 kVA data sheet (illustrative)
Illustrative figures to demonstrate the method — not data for any make or model.
| Field on the sheet | Value given | Check | Result |
|---|---|---|---|
| Prime rating | 125 kVA / 100 kWe at 0.8 PF | 125 × 0.8 = 100 | Consistent |
| Rated current at 415 V | 174 A | 125,000 ÷ (1.732 × 415) = 174 | Consistent |
| Alternator efficiency at full load | 92 % | Engine power to alternator = 100 ÷ 0.92 ≈ 109 kWm | — |
| Engine net prime power (after fan) | 118 kWm | 118 ÷ 109 ≈ 1.08 → about 8 % margin | Adequate; check at site ambient and altitude |
| Fuel at 100 % load | 27 L/h | 27 ÷ 100 kW = 0.27 L/kWh | Plausible |
| Fuel at 50 % load | 14 L/h | 14 ÷ 50 kW = 0.28 L/kWh | Plausible |
| Voltage dip | “Less than 20 %” | For what load step? | Ask — the step size is missing |
| Short-circuit withstand | Not stated | Excitation type is shunt | Ask — sustained fault current may be low |
| Radiator design ambient | 40 °C | Site summer ambient around the set is often higher | Ask for the high-ambient option or derating |
Three questions back to the supplier before this sheet can be compared fairly — and each one could change the decision.
Comparing two sheets fairly
- Put both on the same rating class and power factor.
- Derate both to your site ambient and altitude using each maker’s derating data.
- Compare engine displacement and engine power margin, not just kVA.
- Compare transient performance at the same load step.
- Compare fuel at the load you will actually run — and multiply by the hours you will run.
- Check what is standard and what is optional: PMG, AMF, base tank, high-ambient radiator, anti-condensation heater, battery charger.
- List every missing field as a question — missing fields are not “zero”.
Red flags
- One sheet covering a whole family with “up to” values.
- Fuel figures with no load points or no tolerance.
- Voltage dip with no load step, or no transient data at all.
- Engine power quoted only as gross (before the fan) with a thin margin.
- No reference conditions for the rating.
- No date or revision; model numbers that do not match the quotation.
Data sheet comparison worksheet
| Field | Offer A | Offer B |
|---|---|---|
| Rating class, kVA / kWe, PF | ||
| Rated current | ||
| Governing class | ||
| Engine net power and margin | ||
| Displacement / cylinders / aspiration | ||
| Alternator rating, insulation and rise class | ||
| Excitation type; short-circuit withstand | ||
| Voltage dip at stated load step | ||
| Max single load step | ||
| Fuel at 50 / 75 / 100 % load | ||
| Radiator design ambient | ||
| Noise at 1 m | ||
| Dry / wet weight | ||
| Emission certificate | ||
| Fields missing — questions asked |
Common mistakes
- Comparing a standby rating from one supplier with a prime rating from another.
- Accepting a family brochure in place of the data sheet for the exact model quoted.
- Treating a missing field as acceptable instead of asking for it.
- Comparing full-load fuel figures when the set will mostly run at half load.
What to send us
Send the data sheets or quotations you are comparing, your site’s load and largest motor or UPS, the ambient temperature and altitude, and how many hours a year the set will run. Use the application engineering enquiry, or WhatsApp the details and photographs.
Questions buyers ask
What is the most important field on a generator data sheet?
There is no single one, but the rating basis comes first: rating class, power factor and reference conditions. After that, engine power margin, transient performance (voltage dip and recovery) and part-load fuel use decide most real-world outcomes.
What does governing class G2 or G3 mean?
They are performance classes for frequency and transient behaviour defined in the international generating-set standard. G2 suits general loads; G3 is tighter and is commonly specified for telecom, data and UPS-heavy loads.
Why should I check engine kW when the set is rated in kVA?
Because the engine must drive the alternator, cover its losses and usually the cooling fan. Divide the kWe by the alternator efficiency and compare with the engine’s net power at site conditions. A thin or negative margin means the set may not deliver its rating in summer.
Are fuel consumption figures on data sheets guaranteed?
They are test values under reference conditions, usually with a stated tolerance such as +5 %. Real site consumption depends on load, condition, fuel and ambient. Use them to compare offers and plan, not as a guarantee.
What is short-circuit withstand and why does it matter?
It is the alternator’s ability to keep supplying a high current — commonly about three times rated for ten seconds — during a fault, with excitation support. Without it, fault current can collapse and downstream breakers may not trip.
Can I compare a standby rating from one supplier with a prime rating from another?
No. Convert both to the same rating class first. A standby rating is typically about 10 % higher than the prime rating for the same set, and the two allow very different running patterns.
Related pages
- Datasheets & Brochures
- ISO 8528 Generator Standards
- India Genset Emissions
- Voltage Dip
- Generator Transient Response
- Generator Fuel Consumption
- Generator Protection Relay Architecture
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.